Melamine-formaldehyde resins are extensively used in the manufacture of high pressure decorative laminated plastics because of their transparency, resistance to abrasion, light stability, stain resistance, and relatively low cost. Their use in low pressure laminates, i.e., those molded at pressures of less than 500 psi. has been a problem, due to their low-flow characteristics.
Previous attempts to develop a highly-flowable melamine-formaldehyde resin, useful for low pressure applications, containing large quantities of external plasticizer, such as toluene sulfonamide, methyl glucoside or sucrose, and buffering systems such as guanadine carbonate have not been successful, due to poor laminate surface characteristics, as well as high costs.
Scott, in U.S. Pat. No. 2,908,603, sought to improve the flow characteristics of melamine-formaldehyde resins, for 1,000 psi. laminating applications, by initially forming a methylol melamine reaction product at 85.degree. C. He then added cyclic ethylene urea and heated the materials at 85.degree. C to 95.degree. C, to completely react them to a 25% water dilutability point. Such compositions would have poor storage stability.
Bornmann, in U.S. Pat. No. 3,496,131, attempted to solve storage stability problems of melamine-formaldehyde resins, containing up to 20 wt.% of urea, thiourea, dicyanodiamide, guanidine or benzoguanamine, by adding 0.3 to 3 wt.% of cyanamide. He added the cyanamide to a single step reaction admixture of ingredients at 70.degree. C, and heated at 90.degree. C, to a 100% water dilutability point.
These resinous solutions have not proved to be completely satisfactory for low pressure laminating. For low pressure applications, the resin must be highly flowable, in order to provide a smooth, closed-pore, abrasion resistant film on the impregnated substrate surface. The resin must also set within a commercially acceptable time of about 90 seconds, at about 150 psi. to about 500 psi. In addition, under these quick-set, low pressure conditions, the resin must not all flow into the body of the porous substrate or it will provide a rough surface with a discontinuous film.